1. Field of the Invention
The present invention relates to a control method for a direct power converter wherein an AC voltage is directly converted into an AC voltage having a desired magnitude and frequency, without employing a large energy buffer such as an electrolytic capacitor.
2. Description of the Related Art
FIG. 12 is a configuration diagram showing the main circuit of a matrix converter that is provided with nine bidirectional switches, as a typical example of a direct power converter of the type specified above.
Referring to the figure, numeral 10 designates a three-phase AC power supply, numeral 20 designates a filter which includes a reactor and capacitors, letters R, S and T designate AC input terminals, and numeral 30 designates the matrix converter in which the nine bidirectional switches SW capable of controlling currents bidirectionally are connected between the AC input terminals R, S and T and AC output terminals U, V and W. The bidirectional switches SW are turned ON/OFF, whereby three-phase AC input voltages are directly derived and are converted into three-phase AC voltages of any desired magnitude and frequency.
FIG. 13 is the block diagram of a control apparatus for a matrix converter as is stated in “An Improved Method of Input and Output Waveforms for the Matrix Converter Based on Virtual AC/DC/AC Conversion”, Jun-ichi Itoh, Ikuya Sato, Shigeo Konishi, SPCO2-90/IEA-02-31, 2002; and “A High Performance Control Method for the Matrix Converter Based on PWM generation of Virtual AC/DC/AC Conversion”, Jun-ichi Itoh, Hirokazu Kodachi, Akihiro Odaka, Ikuya Sato, Hideki Ohguchi, Hidetoshi Umida, National Convention Record, IEEJ-Industry Applications Society, pp. I-303-I-308, 2004. In these non-patent documents, control is performed with a matrix converter 30 regarded as a virtual rectifier 30A and a virtual inverter 30B, as shown in FIG. 14. Referring to FIG. 14, reference SWA denotes each of the semiconductor switching elements that constitute the virtual rectifier 30A, while reference SWB denotes each of the semiconductor switching elements that constitute the virtual inverter 30B.
One-leg modulation is employed for the control of the virtual rectifier 30A for the purpose of increasing the utilization factor of supply voltages. As shown in FIG. 13, virtual rectifier control means 41 obtains a modulation signal λREC for the virtual rectifier 30A from phase input current command values IR*, IS* and IT*, so as to generate PWM pulses for a current type PWM rectifier.
The details of the one-leg modulation are stated in, for example, the aforementioned non-patent document “An Improved Method of Input and Output Waveforms for the Matrix Converter Based on Virtual AC/DC/AC Conversion”, and shall therefore be omitted from description here.
On the other hand, regarding the control of the virtual inverter 30B, as shown in FIG. 13, the amplitude command value Vout* of output phase voltages and phase sinusoidal command values VU0*, VV0* and VW0* are multiplied by multiplication means 421 to become the reference signals of the output phase voltages, thereby to obtain output phase voltage command values VU*, VV* and VW*. Besides, in the case of employing one-leg modulation for the control of the virtual rectifier 30A, the fluctuation of a supply frequency component arises in a virtual DC link voltage Ed in FIG. 14. For the purpose of compensating the fluctuation, therefore, the output phase voltage command values VU*, VV* and VW* are divided by the virtual DC link voltage Ed in division means 422, thereby to obtain modulation signals λU*, λV* and λW* for the virtual inverter 30B.
The modulation signal λREC of the virtual rectifier 30A and the modulation signals λU*, λV* and λW* of the virtual inverter 30B as obtained above are synthesized as control commands by control command synthesis means 43, and compared with the triangular wave of a carrier, thereby to obtain the PWM control signals (ON/OFF signals) of the nine bidirectional switches SW of the matrix converter 30. Incidentally, the method for synthesis of the modulation signals and the generation method for the PWM control signals are also omitted from description.
Meanwhile, in the matrix converter, as stated in “An Improved Method of Input and Output Waveforms for the Matrix Converter Based on Virtual AC/DC/AC Conversion”, the maximum effective value of a sinusoidal voltage which can be outputted without distortion becomes 0.866 times the effective value of a supply voltage. In a case where a sinusoidal voltage exceeding the maximum effective value is to be outputted, a large number of harmonic components that are determined by the frequency of the supply voltage are contained in an output voltage. Especially in a case where the supply frequency and the output frequency are different, the waveform of the output voltage changes in every cycle of output.
By way of example, FIG. 15 shows waveforms in the case where, when the effective value of supply line voltage is 200 V, the control command of an output line voltage is set at a sinusoidal voltage whose effective value is 188 V (0.94 times the effective value of the supply line voltages). Referring to the figure, voltages VRS, VST and VTR are the supply line voltages, and −VRS, −VST and −VTR indicate the inverted voltages of the respective supply line voltages VRS, VST and VTR. The output line voltage VUV has had harmonic components ascribable to the PWM modulation removed in order to facilitate illustration.
Here, in a case, for example, where the load of the matrix converter is a motor and where the rated voltage of the motor is equal to the supply voltages, the matrix converter needs to output a voltage effective value, which is at least 0.866 times the supply voltage effective value, in accordance with the rated voltage of the motor. In this case, however, the manner of distortion of the waveform changes every cycle of output as shown in FIG. 15, with the result that nonuniform rotation of the motor and the occurrence of noise from the motor are incurred.
Accordingly, problems arise in the prior art in that the apparatus as a whole becomes expensive because it is necessary to use an expensive dedicated motor with a rated voltage that is low as compared with the supply voltages.